The present invention relates to devices that dispense a pyrethrin-based insect control formulation from a porous wick. More particularly, it relates to providing formulations (and formulation impregnated wicks) for use with such devices, where the formulations include a mix of pyrethrin and an acetylenic hydrocarbon, and as a result reduce the tendency of pyrethrin-based formulations to clog the wick.
Pyrethrum is an insecticide obtainable from a natural plant source. It is typically extracted from a chrysanthemum plant in a form that contains pyrethrin I (see FIG. 1), pyrethrin II, cinerin I, cinerin II, jasmolin I and jasmolin II, usually with some impurities. In some cases one or more of pyrethrin I or II is isolated from the extract, and used separately. More often, the overall pyrethrum extract is used.
Chrysanthemum plants can be grown and harvested in an environmentally friendly (e.g. sustainable) manner, and pyrethrum extract can be obtained from them at relatively low cost. Also, because it is obtained from an essentially natural source, pyrethrum extract has a somewhat wider regulatory and public perception acceptance than a number of more synthetic insect control ingredients do.
To dispense pyrethrum/pyrethrins one typically dissolves the extract in a solvent (e.g. an isoparaffinic hydrocarbon material such as an Isopar), places the resulting solution in a reservoir such as a bottle, extends a porous wick through a cap of the bottle into the liquid, slowly attracts the active/solvent mix out of the reservoir via the wick to an exposed end of the wick, and then drives an active/solvent mix off the exposed end of the wick into the environment (e.g. most typically using a heater system such as that of U.S. Pat. No. 5,647,053). The heater system could be supplemented with a blower if desired (e.g. compare the fan system of U.S. Pat. No. 7,444,833).
Unfortunately, pyrethrum extract impurities tend to react (largely through oxidation and polymerization reactions) to form materials that can clog a dispensing wick. This is in part due to impurities typically found in the extract being oxidized and/or polymerized to clogging materials, and in part because some pyrethrum extract core active components are susceptible to unwanted oxidation/polymerization, again leading to formation and deposition of nonvolatile components in the evaporation zone of the wick that cause wick clogging.
Wick clogging reduces the effectiveness of this type of dispensing device over time, eventually reducing the air release rate of pyrethrin from device, and consequently reducing mosquito repellency. As a result, some consumers may not be satisfied with the performance of conventional pyrethrum-impregnated wicks absent further improvements.
In U.S. Pat. No. 4,798,825 it was disclosed that certain acetylenic substances accelerated pyrethrin decomposition, and thus it was important to remove or modify such materials when using pyrethrin to avoid premature decomposition of pyrethrin.
For example, selective hydrogenation could reduce undesired peroxy species in pyrethrum extract.
In any event, a need exists to address the pyrethrin-related wick clogging concern, without introducing other unacceptable disadvantages (while maintaining the advantages of using pyrethrum).